In semiconductor manufacturing, a fabricated integrated circuit (IC) is usually assembled into a package to be utilized on a printed circuit board as part of a much larger circuit. In order for the leads of the board to make electrical contact with the bonding pads of the IC device, a metal bond, or wire, is formed to make a connection between the bond pad of an IC device and the lead extending to the package lead frame, or solder ball connection to a ceramic or polymeric chip.
As the industry migrates to smaller and smaller size chips, finer pitch wire-bonding is required. The amount of probing damage as a result of the “scrubbing” that is necessary to remove the native oxide on the Al wire bond pads causes a poor wire-bonding surface: Also, the force that is required for “scrubbing” Al bond pads and then consequently the bonding to the damaged area is not suitable for a low dielectric constant, i.e., low-k, interconnect structure. Therefore, a different metallurgy is now required for fine pitch and low-k wire-bond applications.
One proposed noble pad metallurgy that can be used for fine pitch and low-k wire-bond applications is nickel/gold, i.e., Ni/Au. Ni/Au pad metallization provides improved wire-bondability and reliability in such applications.
There are two principal approaches available for plating Ni/Au, i.e., electrolytic and electroless plating. Electroless plating, owing to its distinct advantages such as plating selectivity, ease of processing, and compatibility with the current integration scheme, is used as a preferred method for plating Ni/Au in the present invention.
The process for plating electroless Ni/Au pad metallurgy requires the use of alkaline chemistries that could potentially cause the aluminum wire bond pad to be etched out due to the penetration of alkaline chemistry through the sidewalls of wire bond pads in the current integration scheme. While electroless nickel chemistry is acidic (pH of between 3-5), the most commonly used electroless gold chemistry in the printed circuit board industry is cyanide based, which operates in high pH ranges (pH>10). The other acidic based non-cyanide electroless gold chemistries, while available, are less stable, and present real manufacturability concerns.
To date, no viable means has been developed for protecting Al wire bond pads from such a chemical attack. A method is thus needed for providing a way to prevent the potential chemical attack of Al wire bond pads during the electroless plating of Ni/Au. Such a method should be compatible with existing packaging methods thereby not significantly altering the standard packaging processes.